1. Field of the Invention
The present invention relates to compact zoom lenses for use in digital still cameras and other devices equipped with a solid-state image sensor such as a CCD, and more particularly, to a compact zoom lens suitably used in a mobile camera mounted in or connected to, for example, a mobile telephone, a personal digital assistant (PDA), or a personal computer.
2. Description of the Related Art
Many mobile telephones equipped with a mobile camera having a solid-state image sensor, such as a CCD, have been recently found in the market, and size reduction of lenses for use in the mobile camera has been required. Moreover, a zoom lens capable of zooming from the wide-angle end to the telephoto end has been used more favorably than a single-focus lens in order to add value to the mobile telephones.
However, since the image-taking range is changed by relatively moving the lenses along the optical axis at the zoom lens, it is necessary to allow a space for the lenses to move, in addition to the space in which the lenses are arranged. This hinders size reduction.
Typical digital cameras use a collapsible barrel mechanism in a lens unit to overcome the space problem, but the collapsible barrel mechanism is easily affected by shock. Thus, it is difficult to use a similar collapsible barrel mechanism in a mobile telephone that frequently receives shock due to falling.
In order to ensure a desired zoom ratio in a limited space, all lens units in the lens system can be made movable. However, a motor and a driving mechanism mounted in the mobile telephone are required to be reduced in size and cost, compared when mounted in a popular digital camera, and therefore, it is necessary to minimize the number of movable lens units.
Accordingly, the refractive power of each lens can be increased to ensure a desired zoom ratio while the moving amount of the lens units is limited. However, when the refractive power of the lens is increased, the curvature of the lens is decreased, and correction of aberrations is difficult. Moreover, the edge thickness of the lens is insufficient, and the effective diameter of the lens is also insufficient. When the overall length of the lens system is shortened to the limit, the angle of incidence of light on the CCD differs between the wide-angle end and the telephoto end. While light is sufficiently received by the CCD at the wide-angle end, vignetting is caused by the microlens of the CCD at the telephoto end.
Japanese Unexamined Patent Application Publication No. 2003-177314 discloses a zoom lens suitable for use in a digital camera, which includes a first lens unit formed of one lens having a negative refractive power, a second lens unit formed of one lens having a positive refractive power, and a third lens unit formed of one lens having a positive refractive power. In the zoom lens, the first lens unit and the second lens unit are moved during zooming from the wide-angle end to the telephoto end.
Japanese Unexamined Patent Application Publication No. 2003-177315 discloses another zoom lens including a first lens unit formed of one lens having a negative refractive power, a second lens unit composed of two lenses respectively having a positive refractive power and a negative refractive power, and a third lens unit formed of one lens having a positive refractive power. In the zoom lens, the first lens unit and the second lens unit are moved and the third lens unit is fixed during zooming from the wide-angle end to the telephoto end so that the distance between the first lens unit and the second lens unit decreases and the distance between the second lens unit and the third lens unit increases. Further, focusing is performed by moving the first lens unit.
The zoom lens disclosed in the former publication has a three-unit three-lens structure, and this structure contributes to thickness reduction. However, since only three lenses are used, it is difficult to properly correct aberrations over the entire range from the wide-angle end to the telephoto end. It is also difficult to ensure a zoom ratio of approximately 2× or more.
The zoom lens disclosed in the latter publication can be properly corrected for aberrations because it has a three-unit four-lens structure. However, since focusing is performed by moving the first lens unit forward, there is a difficulty in shortening the overall length of the lens system. From the viewpoints of mechanical strength and reliability with respect to shock such as falling, it is not preferable to move the frontmost first lens unit during zooming or focusing.